1. Field of the Invention
With growing numbers of Internet, mobile telephone, and other network users, network traffic has been increasing, and this, coupled with ever diversifying services from electronic commerce and electronic mail to delivery of moving images, has been increasing capacity needs. To meet such needs, large-capacity networks are indispensable, and the introduction of optical communication networks has been increasing; in particular, wavelength division multiplexing communication networks using wavelength division multiplexing (WDM) technology have been deployed rapidly. In WDM, transmission quality degrades if there occurs a variation in optical power level between wavelength multiplexed optical signals due to variation in characteristics among optical components such as optical fibers, optical amplifiers, etc. In particular, when an optical node (OADM, OXC, etc.) constructed by combining various optical components is used, the variation increases, and it becomes necessary to provide an optical level adjusting function. The present invention relates to an optical level control method, and an apparatus, that have a function to suppress such variation, and that autonomously perform control so as to prevent the occurrence of an optical surge and like phenomenon in the event of a failure or during protection.
2. Description of the Related Art
FIG. 1 shows one example of prior art optical signal level control in a WDM system. A wavelength multiplexed input signal (λ1 to λn) is separated into signals of different wavelengths by an optical splitter 10, and the separated signals are each passed through an optical branching device 12, a variable optical attenuator 14, and an optical branching device 16, and are again wavelength multiplexed by an optical combiner 18 and amplified by an optical amplifier 20.
A photodetector 22 detects the power of light separated by the optical branching device 16 provided on the output side of the variable optical attenuator 14, and feedback-controls the variable optical attenuator 14 through a control circuit 24 to maintain the power level of the light of the corresponding wavelength at a constant level. On the other hand, a photodetector 26 is provided to detect whether the optical signal of the corresponding wavelength has been input normally and also to detect a signal off condition.
As described above, as the prior art optical signal level control requires the provision of two photodetectors for each wavelength, one for feedback control and the other for the detection of a signal off condition, the prior art has had the problem that the amount of circuitry, and the overall cost of the apparatus increase as the number of wavelengths increases.